What extrusion is
Under pressure, an O-ring is forced against the low-pressure side of its groove. If the clearance gap between the mating parts is too large for the pressure and material, the rubber is pushed — extruded — into that gap and is nibbled away.
The three drivers
Extrusion risk rises with a larger diametral gap, higher system pressure, and softer (lower durometer) material. High temperature makes it worse by softening the compound. The gap must include machining tolerances, eccentricity and pressure-driven bore expansion — not just the nominal clearance.
How to prevent it
Reduce the clearance gap, choose a harder compound, or add anti-extrusion back-up rings on the low-pressure side. The extrusion calculator estimates the allowable pressure for your gap and hardness and reports the safety factor.